Underground drilling, such as gas, oil, or geothermal drilling, generally involves drilling a bore through a formation deep in the earth. Such bores are formed by connecting a drill bit to long sections of pipe, referred to as “drill collars” and “drill pipe,” so as to form an assembly commonly referred to as a “drill string.” The drill string extends from the surface to the bottom of the bore.
The drill bit is rotated so that the drill bit advances into the earth, thereby forming the bore. In a drilling technique commonly referred to as rotary drilling, the drill bit is rotated by rotating the drill string at the surface. In other words, the torque required to rotate the drill bit is generated above ground, and is transferred to the drill bit by way of the drill string.
Alternatively, the drill bit can be rotated by a drilling motor. The drilling motor is usually mounted in the drill string proximate the drill bit. The drill bit can be rotated by the drilling motor alone, or by rotating the drill string while operating the drilling motor.
One type of drilling motor known as a “mud motor” is powered by drilling mud. Drilling mud is a fluid that is pumped at high pressure from the surface, through an internal passage in the drill string, and out through the drill bit. The drilling mud lubricates the drill bit, and flushed cuttings from the path of the drill bit. The drilling mud then flows to the surface through an annular passage formed between the drill string and the surface of the bore.
In a drill string equipped with a mud motor, the drilling mud is routed through the drilling motor. The mud motor is equipped with a rotor that generates a torque in response to the passage of the drilling mud therethrough. The rotor is coupled to the drill bit so that the torque is transferred to the drill bit, causing the drill bit to rotate.
Drilling operations can be conducted on a vertical, horizontal, or directional basis. Vertical drilling refers to drilling in which the trajectory of the drill string is inclined approximately 10° or less in relation to the vertical. Horizontal drilling refers to drilling in which the drill-string trajectory is inclined approximately 90°. Directional drilling refers to drilling in which the trajectory of the drill-string is inclined between approximately 10° and approximately 90°.
Various systems and techniques can be used to perform directional and horizontal drilling. For example, so-called “steerable systems” use a drilling motor with a bent housing incorporated into the bottom-hole assembly of the drill string. A steerable system can be operated in a sliding mode in which the drill string is not rotated, and the drill bit is rotated exclusively by the drilling motor. The bent housing steers the drill bit in the desired direction as the drill string slides through the bore, thereby effectuating directional drilling. Alternatively, the steerable system can be operated in a rotating mode in which the drill string is rotated while the drilling motor is running. This technique results in a substantially straight bore.
So-called “rotary steerable tools” can also be used to perform directional drilling. One particular type of rotary steerable tool can include pads or arms located on the drill string, proximate the drill bit. The arms can extend and retract at some fixed orientation during some, or every revolution of the drill string. Contact the between the arms and the surface of the drill hole exerts a lateral force on the portion of the drill string proximate the drill bit. This force pushes or points the drill bit in the desired direction of drilling. A substantially straight bore is drilled when the arms remain in their retracted positions.
Directional drilling can also be accomplished using a so-called “rotary steerable motor” as described, for example, in U.S. Pat. No. 7,389,830, entitled Rotary Steerable Motor System For Underground Drilling, the contents of which is incorporated by reference herein in its entirety. Rotary steerable motors typically comprise a drilling motor that forms part of the bottom-hole assembly, and also include some type of steering means, such as the extendable and retractable arms discussed above in relation to the rotary steerable tool. In contrast to steerable systems, rotary steerable motors permit directional drilling to be conducted while the drill string is rotating. Hence, a rotary steerable motor can usually achieve a higher rate of penetration during directional drilling than a steerable system or a rotary steerable tool, since the combined torque and power of the drill string rotation and the motor are applied to the bit.
Directional and horizontal drilling require real-time knowledge of the angular orientation of a fixed reference point on the circumference of the drill string in relation to a reference point on the bore. The reference point is typically magnetic north in a vertical well, or the high side of the bore in an inclined well. This orientation of the fixed reference point is typically referred to as “tool face,” or “tool face angle.” For example, drilling with a steerable motor requires knowledge of the tool face so that the pads can be extended and retracted when the drill string is in a particular angular position, so as to urge the drill bit in the desired direction.
Tool face, when based on a reference point corresponding to magnetic north, is commonly referred to as “magnetic tool face” (MTF). When based on a reference point corresponding to the high side of the bore, tool face is commonly referred to as “gravity tool face” (GTF). The desired heading for steering during directional and horizontal drilling is usually expressed in terms of GTF, once the initial angle has been established.
GTF is usually determined based on measurements of the transverse components of the local gravitational field, i.e., the components of the local gravitational field perpendicular to the axis of the drill string. These measurements are typically acquired using accelerometers. Acquiring instantaneous measurements of the local gravitational field during rotary drilling is usually not possible, however, because the vibrations of the drill string can be many times greater than one g, i.e., one times the force of gravity.
MTF is usually determined based on measurements of the transverse components of the earth's local magnetic field. These measurements are typically acquired using a magnetometer. Acquiring measurements the earth's local magnetic field during rotary drilling, however, can also present difficulties. For example, a typical drill string can rotate at an angular velocity of approximately 180 revolutions per minute (rpm), or 1,080 degrees per second. The substantially instantaneous determination of MTF under such conditions requires that the components of the transverse magnetic field be measured with sufficient accuracy, and that MTF be calculated in milliseconds. This requirement can place a large, and potentially unacceptable computing load on the down hole data processing equipment used to acquire and calculate MTF. Also, the presence of magnetic material in the drill string proximate the magnetometer can perturb the geomagnetic field, and thereby introduce inaccuracies into the calculation of MTF.
Moreover, as the desired heading for directional and horizontal drilling is usually expressed in terms of GTF, MTF usually needs to be converted to GTF. This conversion typically requires a relatively complex series of mathematical calculations. The need to perform these calculations at a relatively high rate can further increase the computing load the down-hole data processing equipment.
Consequently, an ongoing need exists for methods and systems that permit a down-hole component of a rotating drill string to be activated based on the orientation of the component referenced to GTF, while minimizing the associated computing load.